Television is the principal cause. For a start, people spend so many hours in front of the screen, watching people they are totally unable to interact with directly. To participate, when food ads come on TV it gives them the opportunity to interact by eating too. Yes, they have a remote, they can change channels and they can turn it off, but what are they going to do instead?
They see lives on TV they can never possibly obtain, beautiful homes, beautiful clothes, beautiful locations, impossibly wealthy people who have everything. People with great friends & family and their current problem will be solved by the end of the episode. Most often people watching TV, are in sweats, on ordinary furniture possibly in a trailer park. They can never attain the home, that lifestyle.. so there is no point in trying to fit into the clothes they see on TV. They have no where to wear them to even if they could fit into them. There's minimal dress requirements at Walmart and the mall. Would the trouble they'd have to go to, to lose 20lbs be worth getting off the sofa for anyway?
The other problem is the quality of food these people are eating. Without a doubt, the meat that most people have access to, is battery farm raised. Most likely pumped full of hormones & fed antibiotics to make the animal grow as quickly as possible so farmers can harvest the flesh. No doubt these hormones are passed through to the consumer.
All junk food is full of sugar or corn syrup and salt. No wonder everyone is getting diabetes and heart disease.
These people are ripe with anger at their in ability to change anything in their lives and are thus easy prey for fox news. Washington DC is remote and their single insipid voice unable to be heard by the television. They can shout along side the Fox commentators and agree without ever thinking about what they say, as another form of pseudo participation.
It is likely these people are on some kind of welfare package and so they have no incentive to earn extra so they don't lose their check.